The present application relates to a method for breakdown voltage enhancement and current collapse suppression in normally-off high electron mobility transistors (“HEMTs”), and in particular, to fabrication of aluminum-gallium nitride/gallium nitride (“AlGaN/GaN”) HEMTs using electrode-less drain-side surface field engineering, resulting in a “Low Density Drain” HEMT.
Excessive electric field can create problems in semiconductor devices. (One type of problem is hot carriers, where sufficiently energetic electrons or holes become able to travel through dielectrics; another type of problem is avalanching, where conduction becomes uncontrolled.) Even in devices designed to operate at minimal logic voltages, it is important to ensure that the voltage does not change too sharply at the drain boundary; and in devices which are intended to switch higher voltages, it becomes increasingly necessary to minimize the peak electric field.
Drain engineering has been one of the longest-running sub-areas of integrated device development, going back to the original LDD proposal of 1974. See Blanchard, “High Voltage Simultaneous Diffusion Silicon-Gate CMOS,” 9 IEEE J.S.S.C. 103 (1974). Many techniques have been used to control peak electric field in high-voltage devices, often including various configurations of field plates and non-current-carrying diffusions.
This long-standing development challenge has particular relevance to the relatively new area of enhancement-mode (“E-mode”) III-N HEMTs. Normally-off AlGaN/GaN HEMTs are desirable for microwave power amplifier and power electronics applications because they offer simplified circuit configurations and favorable operating conditions for device safety. However, the normally-off AlGaN/GaN HEMTs usually exhibit lower maximum drain current compared to their normally-on counterparts, especially when the threshold voltage is increased to about +1 V to assure the completer turn-off of the 2DEG channel at zero gate bias and provide additional operating safety. To compensate the reduction in maximum current and achieve the same power handling capability, the breakdown voltage (VBK) needs to be further improved, but preferably not at the cost of increased gate-to-drain distance (which inevitably increases the device size). The use of a field-plate, connected to the gate or source electrodes, can effectively enhance VBK by modifying the surface field distribution. The gate-terminated field plate, however, can introduce additional gate capacitances (CGS and CGD), which reduce the devices' gain and cutoff frequencies. A source-terminated field plate has been used for achieving enhanced VBK and mitigating the gain reduction, but this required a thick dielectric layer between the gate and field plate.
A problem in GaN devices has been the current collapse phenomenon: when the source-drain voltage reaches a level at which impact ionization can occur, the maximum current carried by the device can actually decrease. It has been suggested that this undesirable effect is due to a trapping phenomenon in which mid-gap states are populated by hot electrons.